1. Field of the Invention
This invention relates broadly to film-forming compositions and more particularly to primer compositions. In another aspect, this invention relates to aqueous compositions useful for forming primer coatings for binding high molecular weight emulsion polymerized acrylic polymers to metallic substrates such as galvanized steel.
2. Brief Description of the Prior Art
Coil coating involves the application of a coating composition to continuous strips of a metallic substrate, usually aluminum or steel. The metal is of relatively light gauge and comes to the coating operation in the form of coils from which it is unwound and continuously coated. After coating, the coil passes into a baking or curing oven where it is cured at relatively high temperature for a relatively short period of time. The coated article is then passed to further operations where it is fabricated into desirable shapes such as for home and industrial siding as well as automotive and appliance parts. Compositions for coil coating must be carefully selected because the curing or baking conditions are rigorous. The high temperatures used result in water popping which evidences itself by bubbles or pinholes appearing in the cured film surface. Water popping is observed with many water-soluble coating compositions such as low molecular weight water-soluble acrylics cured with an aminoplast resin. Although the exact cause of water popping is not known with certainty, it is believed that the film begins to cure or crosslink before the last portion of water is eliminated. This residual water cannot diffuse through the tough surface film and collects there as tiny bubbles.
In attempts to overcome the problems of water popping, attempts have been made to use high molecular weight thermoplastic acrylic latices instead of low molecular weight water-soluble acrylics and curing agent. The high molecular weight thermoplastic acrylics do not require crosslinking to develop good coating properties. Unfortunately, these higher molecular weight materials must be prepared and stabilized with one or more emulsifying agents. The emulsifying agent can find its way into the resultant coating where it can sensitize the resultant coating to moisture. The moisture penetrates the film and reduces the adhesion between the film and the metal surface and eventually corrodes the metal surface. This problem is particularly acute with galvanized steel substrates where adhesion between the high molecular weight acrylics and the galvanized steel quickly deteriorates.
In attempts to overcome moisture penetration of these high molecular weight acrylic films, I have experimented with various primer coatings to be applied between the metal surface and the high molecular weight thermoplastic acrylic topcoat. One particular class of coating materials which I have found to provide an excellent moisture barrier to the metal surface are carboxylated polyethers which are described in the aforementioned U.S. Pat. No. 3,960,795. Unfortunately, these materials by themselves do not provide sufficient interfacial adhesion between the primer coat and the high molecular weight thermoplastic acrylic topcoat for commercial applications. However, I have found that if the carboxylated polyethers are blended with high molecular weight water-insoluble acrylic latices such as the type used in the topcoat formulations in specified proportions, commercially acceptable adhesion can be developed while maintaining the good moisture barrier properties of the carboxylated polyethers.
Somewhat surprisingly, the combination of the carboxylated polyethers and the low molecular weight water-soluble acrylics cured with aminoplast does not provide commercially acceptable intercoat adhesion with the high molecular weight thermoplastic acrylic topcoats.